Process for the production of sulphite pulp



Patented Nov. 12, 1929 UNITED STATES' PATENT o FFICE chosen a. mourns, or BERLIN, New; naursnmn, ASSIGNOR 'ro nnown' comm.

OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE rnocnss FOR THE rnonucrron on suLrnrrnruLr No Drawing. Original application filed July 20,v

This invention relates to the production of chemical wood fiber by the sulphite process of digestion, and has for its object the production of a high grade sulphite pulp which is 5 suitable for manufacture into bond, ledger and other writing paper of the highest quality, and which is an excellent raw material for the preparation of fiber of high alpha or resistant cellulose content. \r p 0 For the production of sulphite pulp of high quality, it has 'been'found preferable to digest wood chips in a liquor containing a' soluble sodium salt, as, for example, in a sodium bi sulphite solution having a combined SO content of about 2%, or greater, and a free SO content of about 5%. Such a liquor is particularly effective in penetrating the chips and in reacting with non-cellulo ic constituents thereof, yielding an easily leachable pulp having excellent paper making characteristics. The use of a sodium bisulphite solution as the'digesting liquor, however, entails certain'distinct' disadvantages. Thus, in actual practice I have found that such liquor tends to wear away the brick and cement lining of the usual sulphite digester, and that such lin- 'ing is impaired far more rapidly and severely than when the usual calcium bisulphite liquor containing about 1% combined and 5%. free SO and employed for the production of the usual commercial-sulphite pulp,is employed. This disadvantage incident to the use of a solution of sodium bisulphite may be ascribedto the fact that, whereas acalcium bisulphite liquor assists in the preservation of the lining by depositing'insoluble calcium salts, such as calcium sulphate and sulphite, in the form of a hard-baked scale on the lining during diges- 40 tion, there is no tendency for such scale forcause of the high solubility of the correspond ing sodium salts. Consequently, instead of protecting the lining, a sodium bisu-Iphite solution doubtless reacts with and dissolves the lining material during digestion.

In accordance with the process of the pres ent invention, Dmay employ as the, fiber-lib erating-liquor a sulphurous acid solution of sodium bisuln'hite or other suitable soluble or sodium 'saltscontaining suitable barium or mation from a sodium bisulphite solution, be-

application filed November 1, ism/se ial in. 145,708. Divided and this 1928. Serial No. 294,319.

strontium salts. Such a liquor makes possible the following advantageous features: 1

1. The production of aneasily bleachable, high grade pulp.

2. The preservation of the digester lining, owing to the decomposition of insoluble salts in the form of a protective coating on the lining. v

3. The recovery of the valuable sodium constituent in the spent liquor of digestion, in a condition suitable for reuse in the preparation of fresh cooking liquor.

When a process of fiber liberation such as hereinbefore described is practised in a plant equipped with a recovery system, one may em- 05 ploy a sodium bisulphite solution containing, say, about 2% combined SO, and.con-' taining suitable barium or strontium salts, such as barium bisulphite, in suflicient amount so that when the digester is being relieved in the finalstages of cooking, the insoluble barium monosulphite is precipitated while free S0 is escapingfrom the liquor; Barium sulphite and barium sulphate being of very low solubility, only .a comparatively very small-amount of barium-bearing material need be added to the digested liquor to result in. the precipitation of such salts during 7 the final stages of digestion. Thus,-about10 to 30 pounds of barium carbonate need be add- 80 ed to the digester charge (depending on the 7 size of the digester), the barium carbonate reacting with the sulphurous acid and forming barium bisulphite, which toward the end of digestion is gradually precipitated as barium sulphite and sulphate, as free S0 is releasedffrom the liquor.

, The precipitation of barium-monos'ulphite together with some sulphate, gradually takes place as the free sulphurous. acid concentra- .tion of the cooling liquor is gradually desoluble than the monosul hits, but" as the sulphurous acid which ma1ntains the monophite l s lu i n as bisulphite is decreased inconcentration, a' precipitation'of the'more insoluble monosulphite takes. place; For

' example, if the usual clear calcium bisulphite cooking liquor is boiled and. the tree S liberated permitted to escape, a heavy precipitate of calcium monosulphite results. So,

too, when a clear cookingliquor containing barium bisulphite is employed in, a digester,

as'the liberation'and escape of free SO; proceeds,- the moreinsoluble barium vmonosulphite is precipitated and deposits as scale on the digester lining.

The spent liquor resulting from digestion contains, however, but a small proportion of barium salts, so

to the evaporation andconcentratipn steps without encountering excessive difficulty due to scale formation. O-nlya slight precipitate sulphite liquor,

of barium sulphate [and carbonate is produced in the smelt solution, so that filtration thereof is usually unnecessary. The smelt liquor may then be treated to produce a fresh suflicient suitable barium and sodium-bearing materials being added to compensate for any losses in the cycle.

The barium-bearing material may be barium carbonate, barium hydroxide, or barium sulphate. Barium sulphate remains as such in the digesting liquor, only a relasodium bisulphite solution containing strontium bisulphite as the cooking liquor. Strontium sulphate, however, is more soluble than barium sulphate, so thatif only sulphate is' the salt used in the cooking liquor, it is preferable to employ barium sulphate.

The advantageous features. hereinbefore.

described may be realized when acid sulphit'e phate.

cooking liquors containing sodium salts other than sodium'bisulphite are employed in the production of sulphite' pulp, for instance a cooking liquor consisting of a sulphurousacid solution of sodium sulphate, as disclosed in my U. S. Patent No. 1,427,125, issued August 29, 1922. In such case, the pulping or fiberliberating action is believed to be exercised by the small proportion of sodium bisulphite and sodium sulphite resulting irom the reaction of the sodium sulphate with the free sulphurous acid, the larger proportion or sodium constituent remaining as sodium sul- Such a liquor contains a relatively high concentration of sodium constituent, and is preferably employed in a plant equipped with a recovery system. In accordance that itlmay be subjectedlhlnasmuch with the process of the present invention, such a liquor may be treated approximately to saturation with certain barium or strontium salts. For example, certain basic barium salts, e.g., barium carbonate, maybe added tothe liquor, in which casebarium sulphate is precipitated;or'strontium carbonate may .bility than the other salts, it,may be preferv be. added thereto, in which case strontium sulphite (which is more insoluble than strontium sulphate) is precipitated. Owing to'the fact that barium sulphate has .amuch lower soluable to produce this part cular salt. in such liquor. 1 a i ascertain barium and strontium salt-s may b Successfully employed in car rving out the process of the present invention, I regard them as .chemicalequivalents,

As there 'is'no generic expressionfor both these salts (so far asl aniaware), which does not include calcium salts, by the expression .barium as used in] the appended claims, I

.mean also to include strontium.

' -So far as subject matter is concerned, this is a division ofmy application Serial-No. 14 5.708, filed Novemberl, 1926, which contairis generic claims,

. WhatI claim is:

1'. A process which comprises digesting "raw'cellulosic material in a sulphurous acid solution of a soluble salt containing a barium salt in suflicient proportion therein to precipitate an insoluble barium salt when S0 is released therefrom. and releasing SO from the solution during digestion to cause suchpre-.

cip'itation.

2. A process which comprises digesting raw cellulosic material in a sulphurous acid so:

lution of sodium salts containing barium salts.

3. A process which comprises digesting raw cellulosic material ina sulphurous acid solution of sodium salts substantially saturated with barium salts.

4. A process which, comprises digesting raw cellulosic material in a sulphurous acid solution of sodium salts containing barium salts, concentrating the resulting spent liquor, and smelting and recovering the inorganic content of theconcentrated spent liquor.

5. A process which comprises digesting raw cellulosic material in a sulphurous acic solution of sodium salts substantially saturated with barium salts, separating the resultingspent liquor, and smelting and recovering the inorganic content of the concentrated spent liquor.

6. A process which comprises digesting raw cellulosic material in a sulphurous acid solution of sodium bisulphite containing raw cellulosic material in a sulphurous acid solution of sodium bisulphite containing barium bisulphite in suflicient proportion to precipitate barium sulphite when S is re- 5 leased therefrom. 9. A process which comprises digesting raw oellulosic material in a sulphurousacid solution of sodium bisulphite containing barium bisulphite insufficient proportions to precipitate barium sulphite when S0 is released therefrom, and releasing SO from the solution during digestion to cause such precipitation;

10. A process which comprises digesting raw oellulosic material in a sulphurous acid solution of a soluble salt containing a barium salt in suflicient proportion therein to precipitate an insoluble barium salt when S0 is released therefrom, releasing SO from the solution during digestion to cause such precipitation, concentrating the re .ilting spent liquor, and smelting and recovering the inorganic content of the concentrated spent liquor.

11. A process which comprises digestin raw oellulosic material in a sulphurous acid solution of sodium salts containing a barium salt in suflicient proportion therein to precipitate an insoluble barium salt when S0 is released therefrom, releasing SO from the solution during digestion to cause such precipitation, concentrating the resulting spent liquor, and smelting and recovering the inorganic content of the concentrated spent liquor.

as 12. A process which comprises digestin raw cellulosic material in a sulphurous aci solution of sodium bisulphite containing barium bisulphite in sufiicient proportion to precipitate barium sulphite when S0 is released therefrom, releasing S'O from the solution during digestion to cause such pre cipitat-ion, concentrating the resulting spent liquor, and smelting and recovering the inorganic content of the concentrated spent liquor.

13'. A process which comprises digestin raw oellulosic material in a sulphurous aci solution of sodium bisulphite of about 2% combined SO content and containing barium bisulphite in suiiicient proportion to precipitate barium sulphite when SO is released therefrom.

In testimony whereof I have affixed my signature.

GEORGE A. RICHTER. 

